In general, along with the trend toward the flattening of picture planes of a television set and a monitor, there has been the increased necessity for ensuring mechanical safety of a glass panel for use in a cathode ray tube (hereinafter referred to as CRT) and a CRT including the same. So the glass panel for use in a CRT is usually subjected to a physical strengthening process to improve its mechanical strength by increasing compressive stresses on an inside and outside surface thereof.
In the aforementioned physical strengthening process, when glass is quickly cooled down from a high temperature near the softening point, the surface thereof becomes contracted and solidified, whereas an inner portion thereof still remains in an expanded state (or liquid state) while having sufficient liquidity. As a result, when the temperature of the glass drops to room temperature and reaches a sufficient equilibrium state, a great compressive stress layer is generated on the surfaces of the glass and a tensile stress layer is generated in the inner portion thereof, which results in residual stresses. The intensity of the stresses depends on length of time required for the temperature of the glass surface to drop from the annealing point to the strain point. If the cooling of the glass is carried out quickly, contraction differences between the surface and inner portion of the glass become great and, further, great compressive stresses are generated on the glass surface after the cooling has been completed.
In the physically strengthened panel, the local tensile stress concentration occurs in the inside surface of its corner portion and like. That is, non-uniform stress distribution over the glass panel for use in a CRT occurs, which leads to its deformation.
The following is a description of a conventional physical strengthening process for improving a mechanical strength of a glass panel.
First, a lump of molten glass is press-formed in a mold to make a glass panel to be subjected to the physical strengthening process. Next, cooling air is applied to the press-formed glass panel and, then, the press-formed glass panel is removed from the mold. The removed glass panel is subjected to a stud pin installing process while being naturally cooled down. At this time, residual stresses of hundreds of MPa are generated on the glass panel by the natural cooling. The glass panel with such great residual stresses is very brittle and, thus, the intensity of the residual stresses, which has been generated on the glass panel by the natural cooling, needs to be reduced. Accordingly, the glass panel is reheated in an annealing lehr and maintained below the annealing point for a predetermined period of time, so that the residual stresses generated on the glass panel can be relaxed. At this time, in general, before the glass penal is inputted into the annealing lehr, temperature of most area on the glass panel is dropped below the strain point.
In case the intensity of the residual stresses generated on the glass panel is controlled, as described above, by reheating the panel in the annealing lehr to a temperature below the annealing point and then cooling down the reheated glass panel to room temperature, sizes of the glass panel are changed.
That is, in the conventional physical strengthening process for the glass panel, the skirt portion of the glass panel is cooled down more quickly than the face portion thereof after the glass panel is press formed and before it is inputted into the annealing lehr, so that the intensity of the residual stresses of the skirt portion becomes greater than those of the face portion. Therefore, in the conventional physical strengthening process of the glass panel, the skirt portion, which is cooled down and solidified more quickly than the face portion, is deformed due to its cooling and solidification, and this causes relatively great deformation in the face portion still having viscous liquidity. As a result, the face portion deformation changes an inside surface curvature of the face portion, which leads to inferior characteristics of a screen (or picture) portion of a cathode ray tube.